COURTSHIP IX BIRDS 275 



to the passage of the air through the stiff primary 

 feathers of the wings, or, as some believe, 

 through the outer feathers of the tail. The 



*to 



loud booming or whirring sound made by the 

 nighthawk in his spectacular plunges has already 

 been mentioned, an instrumental music of cu- 

 rious character. 



The drumming of the flicker on a hollow stub 

 or on a roof or chimney-pot is clearly to be 

 classed as instrumental music. I have heard this 

 bird interrupt his spring song to drum and later 

 continue with his vocal music. 



The song of courtship produced by the vocal 

 organs of the bird varies from the rasping, 

 vibrating note of the golden-eye or the aa-ou of 

 the eider, emitted at the height of the dance and 

 display, the harsh scream of the kingbird or the 

 tiS'ik of the Henslow's sparrow to the clear, 

 plaintive whistle of the white-throated sparrow, 

 and the serene, spiritual hymn of the hermit 

 thrush. While the simpler, more primitive songs 

 are given forth only during courtship excite- 

 ment, it is evident that many, especially the more 

 complicated and cesthetic ones, although at their 



